Equisignal radio beacon system



March 30, 1948. w. J. OBRIEN 2,438,573

' EQUI-SIGNAL RADIO BEACON SYSTEM Original Filed March 2, 1942 3 Sheets-Sheet l d INVENTOR MAL/4M rf. 032/5,

BY W ,4 TOR/V5 March 30, 1948. w. ,J. OBRIEN EQUI-SIGNAL RADIO BEAQON SYSTEM Original Filed March 2, 1942 3 Sheets-Sheet 2 INVENTOR N 5. m 0 m M M March 30, 1948. w; J. O'BRIEN I EQUI-SIGNAL RADIO BEACON SYSTEM Original Filed March 2, 1942 3 Sheets-Sheet 3 u V m a .A M h 5 m 4 3 J W 7. ||.H v w M 2/ V0 m 1 w m m m N a a a I w M M 5 w 1x .I u w 1 1 E. x 9.]

Patented Mar. 30, 1948 j EQUISIGNAL RADIO BEACON SYSTEM William J. OBriemLondon, England, assignor to The Decca Record Company, Limited, London, England, acorporation of Great Britain Original application March 2, 1942, Serial No.

432,948. Divided and this application Novemher 5, 1943, Serial No. 509,024. In Great Britain March 1, 1943 8 Claims. (01. 343- 107) 1 My invention relates to a radio beacon system and has particular reference to an equi-signal ty e of radio beacon which finds particular utility when employed as a navigation aid for vehicles and conveyances, particularly aircraft.

This is a division of my copending application Serial No. 432,948, filed March 2, 1942 now Patent No. 2,406,396 issued August 2'7, 1946 and entitled Equisignal radio beacon system. i

The equi-signal type of radio beacon system is that which is at present employed in the United States De'partmentof Commerce airways radiorange beacon for establishing the transcontinental air routes followed by the commmerclal air transport companies. This type of system generally comprises a. group of radio transmission antennae so arranged and :so operated that the course along which it is desired to navigate an aircraft comprises the locus of points of equal signal intensities as regard separate signals of like frequency emanated from the antennae. In general, these separate signals are reciprocally and distinguishably keyed "oi-I and on, the

' conventional arrangement being that in which one signal is keyed with the International Morse Code character for A while the other is alternately keyed N If the aircraft is on course the intensities of the two signals are equal and the off periods of one coincide with the on periods of the other so that a steady and continuous signal is received. If the plane is off course, one of the signals will predominate over the other and the keying of. the

signals will be apparent. The direction in which:

the plane has drifted from the course is indicated by whether the A or N signal predominates.

The present systems while of great assitan-ce in the navigation of aircraft, nevertheless are characterized by certain disadvantages and difiiculties. For example, the present systems do not provide a sufficiently sensitive indication to permit their successful application tolong range control, as for example, the guiding of bombing providean equi-signaltypeof. radio beacon system which overcomes the above noted disadvantages by providing for increased sensitivity of indication.

It is a still further object of my invention to provide a radio beacon system of the character set forth hereinbefore which also includes a novel signal modulating arrangement which avoids the necessity of keying the beacon transmitters.

It is also an object of my invention to provide a radio beacon system of the character set forth in the preceding paragraph which includes an improved type of visual indicator for apprising the pilot of a vehicle or conveyance of his location with respect to the course intended to be followed.

Other objects and advantages of my invention will be'apparent from a study of the following specifications, read in connection with the accompanying drawings, wherein:

Fig. l is a diagrammatic view illustrating by means of curves drawn on rectangular coordinates the relative signal intensities produced by two continuously operated radio transmission antennae with a periodically reversed signal in troduced from a central antenna;

Fig. 2 is a diagrammatic view illustrating the mode of operation of the three antennae to produce the field patterns illustrated in Fig. 1, and illustrating also the manner in which that mode of operation serves to provide an indication of the location of a vehicle with respect to the course intended to be followed;

Fig.3 is a wiring diagram'illustrating diagrammatically a novel form of transmitting equipment for defining equi-signal courses and for producing a somewhat difierent type of indication as to the location of a vehicle with respect to one 0 the equi-signal courses; 7

Fig. 4 is a rectangular diagram illustrating the wave form of the modulating voltage applied to the signals emanated by the central antenna of planes in war time to an'objective to be bombed situated five or six hundredmlles distant. The sensitivity of the present systemcan be increased by increasing the antenna spacing. There are, however, practical limitations on how far apart the antennae may be successfully spaced and,

the system illustrated in Fig. 3 and illustrating also the sine wave components which produce that particular form of modulating voltage;

Fig. 5 is a rectangular diagram illustrating the modulated radio signal which is emanated from the central antenna;

Fig. 6 is a diagram illustrating the radio frequency signal which is emanated by the two spaced antennae of the system;

Fig. 7 is a diagram illustrating the result of combining the radio frequency voltages illustrated, respectively, by Figs. 5 and 6 and rectifying this result;

Fig. 8 is a view similar to Fig. 7 but illustrating the rectified wave form resulting from the condition in which the amplitude of the radio frequency illustrated by Fig. 6 is equal to the maximum amplitude of the radio frequency signal illustrated by Fig. 5.;

Fig. 9 is a diagram illustrating the change in wave form resulting from shifting one of the sine wave components illustrated in Fig. 4 relative to the other;

Fig. 10 is a schematic wiring diagram illus trating the receiving apparatus for receiving the signals produced by the systemillustrated in Fig. 3 and including a cathode ray tube as the visual indicating instrument by means of which the pilot of an aircraft is apprised of the location of the craft with respect to the selected course;

Fig. 11 is a schematic wiring diagram illustrating a somewhat different form of receiving apparatus in which a galvanometer is employed as the visual indicating instrument;

Fig. '12 is a diagrammatic representation of the fluorescent screen of the cathode ray tube and illustrating the form of the pattern produced thereon by the electron stream when the receiving apparatus is situated far to the left of the intended course;

Fig. 13 is a view similar to Fig. 12 but illustrating the pattern produced when the craft is moved nearer to the intended course;

Fig. 14 is a view similar to Fig. 12 "but representing the on course indication produced by the receiving apparatus;

Fig. 15 is also a View similar to Fig. 12 but illustrating the character of indication provided when the vehicle is moved somewhat to the right of the intended course; and

Fig. 16 is .a viewsimilar to I2 and illustrating the indication produced when the vehicle is far to the right of :the course intended to be fol- 'lowed.

Referring to the drawings, I have illustrated in Fig. 1, by means of the dashed line ,A, .B, the field pattern produced by .a continuous operation of two antennae spaced two Wavelengths apart with the energy radiated from these antennae. in phase opposition to each other. The solid line in Fig. 1 illustrates the field strength pattern resulting from introducing radiation from a centrally positioned antennaC, which radiation is so phased as to lead the radiation from antenna A by 90 electric-a1 degrees. Similarly, the dotted line curve vin Fig. 1 illustrates the field pattern resulting from so operating antenna C as to cause the signals to lag the A signals by 90.

It will be noted that if the solid line and dotted line patterns are produced alternately as by periodically reversing the phase of antenna C, there are defined at the intersection of the solid and dotted lineseight equisignal courses which are characterized by high sensitivity and accuracy. It'will be further noted that since the solid and dotted curves parallel each other, the difference in signal-intensity resulting from a movement of the vehicle to one side of the selected course will be limited to a reasonable maximum.

Fig. 2 is intended to illustrate the mode of operation of the antennae to produce this type of field strength pattern and illustrates also the manner in which the signals provide an indication to the pilot hi the vehicle as to his location with respect to a selected course. .2857 referring to the upper portion of Fig. 2, it will be noted that antennae A and B are operated continuously, whereas antenna C is so operated as to have its phase periodically reversed. This periodic operation of antenna C is so arranged that the periods of operation in one phase are substantially three times as long as the periods of operation in the other phase.

Along the equi-signal course the signals resulting from each vof the two types of operation of antenna C will be equal so that the signal heard :by the pilot .of :the vehicle will be such as that represented by the straight line 25 in Fig. 2. If, on the other hand, the vehicle wanders to one side of the selected course, the signals resulting from operating antenna C in such fashion as to lead the signals emanated from antenna A will predominate over those resulting from the opposite operation of antenna C. This predominance is illustrated in the lower portion of Fig. 2 in that part disposed to the left of the wavy break line 26. This produces a signal which the pilot of the vehicle bears and interprets as comprising a series of spaced dot signals.

When the vehicle wanders to the opposite'side of the course, the conditions just described are reversedso that the signals of longer duration predominate :over the signals of short duration as is illustrated by that-part of Fig. .2 lying to the right of the wavy break line 27. This producesa signal which the pilot interprets as com- .prisinga series of spacedda'sh signals.

I have illustrated in 3 an improved form of radio transmitting apparatus adapted particularly to the production of course-determining signals for an e'qui-signal beacon system such as that just described.

The apparatus accordingly includes transmitting antennae A, B and C which are geographically positioned as described hereinbefore. Antennae A and B are each coupled'through suitable loading devices 30 and impedance matching transformers 3| to a suitable source 32 of radio frequency energy, a suitable transmission line such as a coaxial cable 33 being employed to convey the radio frequency energy from the source 32 to the .couplingdevices 3 I.

Antenna C is similarly coupled to a power amplifier '34 which is adapted to be excited by the signals produced by the signal source 32 and modulated by a modulator indicated generally by the reference character 35. The modulator 35 preferably includes an input transformer 36, the primary of which is connected as by means of conductors 31 to receive energy from the signal source 32. The. secondary of the transformer 36 is preferably center tapped and grounded through a grid resistor 38, the ends of the secondar Winding being respectively connected as by conductors 39 and 40 to the grids of a pair of screen grid modulator tubes 41 and 42.

The secondary of the transformer 36 is preferably tuned to the frequency of the signal source by means of shunt connected condensers G3 and 44 and the requisite grid bias for the tubes 4| and 42 may be obtained by connectin the cathodes thereof to ground through a cathode bias resistor 45 shunted by a suitable by-pass condenser 46.

The plates of the tubes 4| and 42 are connected in parallel. by means of a conductor 41 through the tuned primary of a coupling transformer 48 to a source of plate potential indicated schematically by the character 13+. The coupling transformer 48 is connected to the input of the power amplifier 34 so that signals produced by the source .32 are modulated in akmanner'to. be described hereinafter by the tubes 4|and42 and are impressed upon the antenna The screen grid elementsof each of the tubes 4! and 42 are connected respectively to opposite ends'of a modulating secondary winding provided on a modulating transformer 49. This Winding is center tapped and connected to a suitable source of positive screen potential indicated schematically bythe character 5+. The primary of the modulating transformer Q-Q'is connected to a suitable source of modulating signal. 7

It will be noted that the grids of the tubes 4i and 42 are connected in push-pull relation to each other so that the potentials impressed thereon areof opposite phase. The plates of the tubes 4i and 42 are connected in parallel with the result that whenever the amplification of the tubes is equal, no signal will be impressed across the coupling transformer 48.

An alternating current modulating signal al plied to the modulating transformer 49 will operate to shift the screen potentials applied to the screen grid elements of the tubes 4| and t2 simultaneously and in opposite directions so as to oppositely vary the amplification of these tubes. As the amplification of the tubes is shifted from the balanced condition to that, for example, in which the amplification of the tube ti exceeds that of the tube 42, a signal will appear across the coupling transformer 4 3, the amplitude of which signal is proportional to the magnitude of the potential shift of the screen grid elements. If the potential applied to the screen grid elements is oppositely shifted a like amount as is the case during the negative half cycle of an alternating current modulating signal, anequal signal will be produced across the coupling transformer 48. This signal will, however, have a phase relation opposite to that of thesignalproduced in the previously described condition.

The modulator 35, therefore, operates to vary the amplitudes of the signals produced by the an tenna C so as to produce a radio frequency signal envelope having the sameshape as the wave form of the modulating signal impressed on the modulating transformer 19. In addition to this conventional modulating feature, however, the modulator 35 serves to actually reverse the phase of the radio frequency signals produced each half cycle of the modulating signal.

With the foregoing general considerations in mind, reference is had to Fig. 4 which illustrates the preferred type of modulating signal applied to the transformer 48. I This signal is a complex wave formed by adding a fundamental sine wave Fig. 4 to a second harmonic signal of half the amplitude of the fundamental signal as is represented by the dotted line 5! in Fig. 4. When the phase of the fundamental and the second harmonic are adjusted with respect to each other as is illustrated in Fig, 4 wherein the fundamental 50 is illustrated as bearing the multiple phase quadrature relation to the second harmonic signal 5!, the resulting signal produced by the addition of this fundamental and second harmonic comprises a complex wave form such as that illustrated by the solid line 52 in Fig, 4. The complex wave form illustrated by the solid line 52 in Fig. 4 is the modulating signal which I prefer to supply to the modulating transformer 45.

The signal emanated from antenna C resulting from this modulation of the radio frequency signal is illustrated in'Fig. 5. wherein the dotted line 6 53 is intended to illustrate the radio frequency signal and the solid lines 54 are intended to illustrate the envelope defined-by the varying amplitude of the radio frequency signals 53. The frequency indicated in Fig. 5 by the wavelength of the radio frequency signals has been greatly reduced for the purpose of clarifying the drawing and the description. It will be realized that in practice the radio frequency signal has a frequency so high with respect to the modulation frequency that it would be impossible to accurately represent the frequency difference in a figure such as Fig.5.

Recalling now the preceding description of the radio beacon system of my invention, it will be observed thatantennae A and B are continuously operated in phase opposition to each other so that the equi-signal course"defined by the two field patterns resulting from the introduction of a periodically reversing radiation from antenna C lines along a line of zero signal as regards the vector sum of the A and B signals.

Whenever the vehicle wanders from the equisignal course, the vector sum of the A and B signals becomes a finite value and this condition is illustrated in Fig; 6 wherein the sine wave 55 the receiving apparatus mounted on the vehicle is, of course, the vector sum .of the A, B and C signals.

It will be noted by comparing Figs. 5 and 6 that duringthat portion of the transmission of a given cycle lying between the points marked on Fig. 5 with the reference characters 56 and 5'1, the A and B signals represented by Fig. 6 are in phase opposition to the C signal represented by the dotted line 53 in Fig. 5. During that portion of the cycle lyingbetween the point 51 and the point marked 58, the A, B signals are in phase with the C signal. This periodic reversal of the phase relationships occurs each half cycle where reference is made to thev frequency of the modulating signal. The result of detecting as by rectification of the combined A, B and C signals is illustrated by the solid line curve 59 in Fig. 7.

If the vehicle wanders farther from the equisignal course than is represented by Fig. 6, the amplitude of the A, '3 signal increases correspondingly. Fig. 8 illustrates by the solid line curve 60 the rectified signal produced when the vehicle has wandered from the course sufiiciently far for the amplitude of the A, B signal to be equal to the maximum amplitude of the C signal. It will be obvious, of course, that when the vehicle is on course, no A, Bjsignal is received because the amplitude of that signal is zero and the C signal is the only one which is received. Detection of the C signal will produce a curve corresponding exactly to that portion of the envelope curve 54 which is disposed above the longitudinal axis represented by the line 6| in Fig. 5. If, on the other hand, the vehicle were to wander to the opposite side of the equi-signal course, the phase of the A, B signal would be precating visually to the ,pilot of the vehicle his position with respect to the equi-signal course. I have illustrated in Fig. one form of receiving and indicating equipment which is particularly adapted to providing this form of indication.

This receiving apparatus may include a substantially conventional radio frequency receiver or amplifier 64 equipped with suitable signal pickup facilities such asan antenna connection 95 and a ground connection 66. The radio frequency output of the receiver is preferably coupled as by means of a conductor '6"! to the plate of a diode or rectifier tube 68. The cathode of the rectifier tube 68 is preferably grounded as at B9 and connected through a cathode resistance 10 to the other output terminal of the receiver 64 as by a conductor H. The resistance 19 is preferably icy-passed with a shunt connected by-pass condenser Y The rectifier 68 operates to produce across the resistance 10 a, direct potential whose magnitude varies in accordance with the shape of the modu lation envelope of the radio frequency signals received by the receiver 64. The potential of the conductor H with respect to ground, therefore, conforms to the wave shapes illustrated in Figs. 7 and 8 previously described and also to the upper half of the modulation envelope 54 which is illustrated in Fig. 5.

This varying voltage is coupled by conventional resistance coupling including a coupling condenser 13 and a grid resistance 14 to the input of an amplifier tube 15. The output of this tube is coupled by means of a plate resistance 16 and a coupling condenser 11 to a switch terminal 78 which is adapted to be contacted by a movable switch armature 19 which is in turn connected to the input of an amplifier tube 88. The grid circuit of the amplifier tube 89 preferably includes also a grid resistance 8|.

The grid bias for the tubes and 80 may be provided as by means of grid bias batteries 82 and 83 connected. between the lower ends of the resistances 14 and BI and round.

The output of the second amplifier tube 89 is coupled through a resistance coupling unit including a coupling condenser 84, a plate resistance 95 and an output resistance 86 across the vertical deflection. plates 8'! of a cathode ray oscillograph tube 88.

By this means the fluorescent spot produced upon the screen of the cathode ray tube by the normal operation of this tube is caused to move vertically back and forth across the screen of the tube in accordance with the amplitude variations of the detected signal as represented by Figs. 5, '7 and 8 previously described. The cathode ray tube is preferably so operated as to simultaneously and repeatedly move the cathode spot from left to right across the cathode, ray tube screen so as to cause the. combined movements of the spot to trace upon the screen of the tube a pattern corresponding exactly to the wave form of the detected signal.

This may be accomplished by applying across the horizontal deflection plates 89 of the cathode ray tube 88 a control voltage having a saw-tooth wave form; that is, the voltage is caused to build up at a constant rate from a minimum value to a predetermined maximum value and then fall almost instantly to the predetermined minimum value. This type of control volt-age is generated by a saw-tooth oscillator, employing a grid controlled gas discharge tube 90. A condenser 9| is connected across the plate and cathode of the tube and the plate of the tube 90 is connected to a suitable source of direct potential such as is indicated by the legend B+ through a conductor 92 in which is included a fixed resistance 93 having a relatively high ohmic value. The voltage which appears between the plate of the tube 99 and ground is preferably applied across the deflection plates 89 as by grounding one of these plates and connecting the other to the plate of the tube 90 through a coupling condenser 94. The ungrounded deflection plate 89 is preferably grounded through a leak resistance 95.

In operation, direct current flows from the source B+ through the resistance 93 and into the condenser 9| so as to charge up the condenser 9l. The time required for charging the condenser 9| to a predetermined voltage is determined by the capacity of the condenser 9| and the resistance of the series resistor 93. As the voltage across the condenser 9| builds up, the cathode ray spot on the fluorescent screen of the cathode ray tube 88 is caused to progress across the screen at a uniform rate until the voltage across the condenser 9| builds up to a value sufiicient to ionize the gas in the grid controlled gas discharge tube 90. When this condition obtains, a low resistance conductive path is formed between the plate and cathode of the tube 99, serving to completely discharge the condenser 9! almost instantly. The resulting fall in voltage between the plate and cathode of the tube 99 interrupts the discharge within the tube and allows the Voltage across the condenser 91 to again start the slow building up process.

I prefer to select values of capacity and resistance for condenser 9| and resistor 93 which will make the time of this condenser discharge cycle equal to two cycles of the signal which is applied to the vertical deflection plates 81. The sweep frequency generated by the tube 99 may be synchronized with the signal which is applied to the vertical deflection plates 81 by connecting the grid of the tube 99 to the ungrounded deflection plate 81 through a coupling condenser 96 and a coupling resistor 91.

The system just described operates, therefore, to produce on the cathode ray tube screen a continuous trace faithfully reproducing the wave form of the detected signal which is picked up by the receiver. The form and configuration of this trace may be used to readily indicate to the pilot of a vehicle his position with respect to the equi-signal course.

Fig. 14 represents the appearance of the oathode ray tube screen when the vehicle is on course. It will be noted that this trace is characterized by four positive peaks indicated in Fig. 14 by reference characters 98, 99, I00 and I91. A comparison of Fig. 14 with Fig. 5 will indicate that the trace of Fig. 14 is an accurate representation of the positive half of the modulation envelope produced by detecting the signals received when the aircraft is on course.

If the aircraft wanders to the left of the course, the trace on the cathode ray tube screen will change to a form such as that illustrated in Fig. 13, this trace being the equivalent of the rectified signal illustrated in Fig. '7. Attention is directed to the fact that the peaks 99 and I00 are closely adjacent and that the on course condition is indicated by the equality of value of these peaks. Since the peaks 99 and I0!) are closely adjacent "and since the on course indication illustrated in Fig. 14 is symmetrical about a vertical center 9:? line, it becomes extremely easy to detect slight deviations from thecourse by the destruction of the vertical symmetry of the figure and the resulting difference in heights of the peaks 99 and I00. 7 I V Fig. 12 illustrates the trace resulting from the drifting of the vehicle still farther to the left of the course, while Fig. 15 and Fig. 16 illustrate successive deviations of the vehicle to the right of .the equi-signal course.

It will be noted that when the vehicle has moved to the left of the course, the left-hand one of the two peaks 99 and I is lowered, or conversely stated, that the right-hand one of the two peaks is the taller. When the vehicle wan ders to the right of the course, the converse situation obtains and the left-hand one of the two peaks 99 and I00 is the taller. Thus the direction of deviation of the vehicle from the selected course is also indicated, the taller of the two peaks indicating thedirection of corrective travel in order to reach thedesired course. a

In the system just described it is highly desirable that the'outside antennae A and B be so operated as to :provide: a signalintensity which is considerably in excess of the signal'intensity produced by thecentral antenna C; The greater the difference between the intensitiesof these signals, the greater is the sensitivity of indication afforded by the traces formed on the cathode ray tube screen. For example, if antennae A and B are so operated that each of them produces a field intensity at the location of the vehicle of fifty microvolts per meter, the intensity of the A, B signals will vary from zero when the vehicle is on course to one hundred microvolts per meter as a maximum. If the antennae A and B are spaced two wavelengths, this maximum will be-realized when the vehicle; has wandered from the course. .When the vehicle is but 1 off course, the field strength ofthe A, B signals will be ten microvolts permeter. I

If at this time the maximum instantaneous field strength of the modulated signal received from antenna C'is tenimicrovoltsper meter, the trace producedon the cathode ray tube screen will be that illustrated by Fig. .12 or Fig. 16 so that the change in visible'indication produced by the vehicle in wandering from an on course location to alocation 1 off of the desired course will produce a change in the. indicated pattern such as that represented in the change from Fig. 14 to Fig. 12.

If, on the ,other hand, thefield strength produced by the central antenna C were thirty microvolts per -meter' instead of the ten assumed in the preceding example, the change produced by the movement of the vehicle from the course to a location l .from the course would be such as that illustrated by thechange from Fig. 14 to Fig. 13. Obviously the sensitivity of indication is less in this second example.

From the above examples an idea ofthe sensitivity of the indicating system of my invention may be obtained. ".If the change from Fig. 14 to Fig. 12 represents only a 1 deviation from the selected course, it will be apparent that a discernible difierence in the height of the two peaks 99 and I00 will be produced by an almost negligible deviation of the vehicle'from the indicated course. fl:

If the vehicle is operating at a great distance from the beacon transmitting stations, the noise level produced by staticand other extraneous disturbances may be sufficiently high to make it 10 diflicult to ascertain the relative heights of the peaks 99 and I00. I

In this casethe switcharm 19 illustrated in Fig. 10 is thrown to its second position. This arm is also connected mechanically to a second switch I02 so arranged as to be closed when the switch arm I9 is thrown to theaforesaid other position. The switch I02 is included in a circuit couplinga suitable radio frequency oscillator I03 to the primary winding of a coupling transformer I04. The secondary of-this; transformer is connected in series in the cathode-circuit of the rectifier tube 68. The radio frequency output of the receiver 64 is thus mixed with the radio frequency output of the oscillator I03."The frequency of the oscillator I03 is so adjusted that the diiferencein frequency between, these two radio frequencies is preferably of an audible frequency. r

This audible frequency appears'as an alternating voltage across the cathode resistance I0 and is modulated by the modulation envelope of the received signal. This modulated beat frequency is preferably applied to the grid of an amplifier tube I05 through a suitable condenser and resistance coupling I06. The outputLof. the tube I05 is connected to a coupling transformer I01 whose primary and secondary windings are sharply tuned to the beat frequency which is. amplified by the tube I05. Theztuned secondary of the transformer I0'I is connected with a switch point I08 which is contacted by the switch arm I9 when it is thrown to its alternative position. The modulated audible frequency is thus applied to the amplifier tube80 and to'the vertical deflection plates of the cathode ray tube 88 so that there is produced on the screen of the cathode ray tube a trace similarto those illustrated in Figs. 12 through 16' but comprising actually the trace of the modulated audible frequency which is impressed on the vertical deflection plates.

The sharply tuned transformer I01 operates to exclude a high percentage of the noise, static, and extraneous signals which are picked up by the receiver 64 and this latter system thus serves to materially extend the.useful range of the indicating system. e In case the noise level is so high that the arrangement just suggested fails to produce understandable indications on the cathode ray tube screen, recourse may be had to energizing the central antenna C intermittently. During the periods of non-operation of thecentral antenna, the trace produced on the cathode ray tube screen will be such as that illustrated by Fig. 6, the amplitude of this trace being zero when the vehicle is on course. Whenever the trace on the cathode ray tube screen changes during the periods of non-operation of the antenna C from a'straight horizontal line to a trace such as that illustrated by Fig. 6, the pilot of the vehicle is apprised of the fact that he has wandered from the desired course. During periods of operation of the central antenna, the trace on the cathode ray tube screen will be such as that illustrated in Fig. 12 or Fig. 13 and a determination of which 1 of the peaks 99 or I00 is the taller will apprise the pilot of the direction in which he must pilot the vehicle in order to come back to the desired course. y

In Fig. 11 I have illustrated an alternative form of receiving and indicating equipment which may be used with the beacon system described in connection with Figs. 3 through 8. In this form of the invention asuitable radio frequency receiver or amplifier l 09is coupled to an antenna H and ground connection H! toreceive and amplify the signals emanated from antennae A, B and C, the output of the amplifier I09 being applied across a diode or rectifier tube I I 2 in the cathode circuit of which is placed a cathode resistance H3 by-passed'by a suitable by-pass condenser H4.

The detected signal corresponding to the modulation envelope thus appears across the resistor H3, the ungrounded terminal of which is connected through a'co'ndenser H5 to the grids of a pair" of'amplif yiiig'tubes H6 and H1, these grids being connected in parallel and grounded througha grid resistance and a grid bias cell or battery in the conventional manner.

The outputs ofthe. tu es Hit-and m are fed, respectively, to timed coupling transformers H8 and H9. The secondaries of these two transformers are also tuned and are. connected in series as by means-01 a conductor I20. One free end of the series connected secondaries is grounded as at HI and the other is connected as by means of a conductor I 22 to the grid of an amplifying tube I23. Suitable operating bias for the amplifier tube I23 may beobtained by means of a cathode bias resistance I24 suitably by-passed as by means "ofcondenser. I25.

'The transformer". H8. is. tuned tothe second harmonic component of' the modulating frequency. while the transformer H9. is tuned to the fundamental. component of the modulating frequency. One onboth of. the. transformers is, however, slightly. detuned so. as to produce a shift in'the phase. ofv the signal passed through one transformer different. from the phase shift producedon the signal passed through the other transformer." difference in. phase shift is so adjusted as. to change the relationshi be? tween the fundamental andtheharmonic illustrated in, Flat. to. that which is illustrated in i -1 wher in. hedottedlineala r presents the second harmonic. component. of. the signaland the do -dash line .50creprcsents the fundament l comp nent. of; the signal,

i 9. illustrates. the. phase relat on of the signals at the output terminals of thdtiansformers H8, and I I9; so that the recombining of these signals by the seriesconnection of the secondaries of-thetrcnsiormers HBand H9 produces a wave form such as that illustrated by the solid n 52a in Fi 9- It will be, notedthat thiswave form is character ed by having a positivepeak of relatively low amplitude and long duration and a negative peak of relatively. high amplitude and short duration. This is the condition which obtains when the vehicle is to one side of the selected course. When the vehicle is on the opposite side of the selected course, the phase of the high frequency component is reversed with respect to the phase of the low frequency component so that the resulting signal applied to the grid of the tube I23 is the reciprocal to that which is illustrated in Fig. 9 and ischaracterized by having a relatively high positive peak of short duration and a relatively low negative peak of long duration.

The output ofthe tube I 23 is applied across the primary of a transformer I26, the secondary of which is preferably tapped at the center and grounded as by a conductor I21. The free ends of the secondary windingof the transformer I26 are connected, respectively, to the grids of a pair of amplifier tubes I28.and I20.. The plates of these tubes are each connected to a suitable source of plate potential represented by the legend B+ through:plate'resistances I30 and I3 I, which resistances are, respectively, shunted by high capacity condensers I32 and I33. The plates of the tubes I20 and I29 are, furthermore, interconnected directly through a low resistance galvanometer l 34 which is preferably provided with a center zero scalef Each of the tubes I28 and I23 is biased to-practically'cut off as by connecting the cathodes thereof through a common cathode bias resistor I35. When so biased, neither of the tubes I28 or lit! will draw an appreciable plate current until a positive potentialis applied to the grids thereof. b

When the vehicle is bfi course in such direction that the signal applied to the primary of the transformer I26- is such as that illustrated by the solid line 52a in Fig. 9, a voltage having this precise wave form will be applied to the-grid of the tube I28and an exactly similar voltage but of opposite phase will be applied to the grid of the tube I29. Thusfduring the long duration positive halfcycle of the receivedsignal, the tube I28 will draw plate current, while during the short duration negative half cycle the tube I29 will draw current. A voltage drop will be produced across the resistances I30 andv I 3| which is proportional to the magnitudes of the currents drawn by the tubes, and-the condensers I32 and I33 will be similarly-charged,

The high capacitypf-thesecondensers and the relatively high ohmic-value-which is preferred for the resistors I 30 and I31 serve to maintain the condensers I32 and I33 substantially charged during the half cycle in which the associated tube is not drawing. any. plate current. Since the magnitude'ofrthe negative half cycle is considerably in. excess of. the value of the positive peak, the plate current drawn-by the tube I29 will-exceed that whichlisdrawnby the tube I28. Thus the voltage which is produced across the resistance I3! will exceedthe voltage which is produced across the resistance I30. Since these voltages are. opposed to each other the meter I 34 will measure :thedifference. in voltage and the direction. of deflection of the meter I34 will indicatethepolarityQof this difference in voltage.

If thevehicle wanders to the opposite side of the selectedcourse, theconditions with respect to the tubes. I 28;: and. 129 will be just reversed from thatabove .describedbecause of the reversal of phase of-t'ha signal which is, applied to the rids of each of these tubes. Thus the voltage across theresistancewflwill exceed that across the resistance I31 andthe needle of the galvanometer 13d will be. deflected in. the. opposite direction. Thusthedirction of.deflection of the meter I34 wiliiserveyto lndicatethe location of thevehiclewith respectto .the selected course.

When thevehicleiis fon course," the wave form which is applied .to. thegridsof the tubes I I6 and II! will be such. astdhave no low frequency component, to be passed through the transformer H9. sothat hereewill be no difference in the magnitude. of the; positive and negative peaks of the si nalapplledtoithe tubes I28 and I29 with the e ult.t atethc,voltag stacross the resistances I30 and I3-I will be equal and no deflection of the meter :I34-will bevproduced. The center position of the meter I34 thus indicates the fact that the vehicle is On the selected course.

The system above-described provides a distinct advantage over theconventional methods of r ting radio range beacon transmitters wherein o'neor all of the transmitters are keyed 13 off and on in that the equipment and apparatus required to modulate the signal emanated from the central antenna is much simpler and less likely to get out of order than is a mechanical switching arrangement such as is required for operating the transmitting equipment intermittently.

Furthermore, the relatively simple wave form of the modulated signal and its relatively low frequency serve to limit the side bands of the transmitted signal within narrow limits, thus permitting a number of beacon systems of this type to be operated on closely adjacent frequency channels. The mechanical keying system tends to produce square-topped Waves which, as is Well known, comprise an infinite series of harmonics with the result that the transmitted side bands have great width as measured in terms of frequency.

It will also be observed that I have provided a unique and novel means for visually indicating to the pilot of a vehicle his location with respect to a desired course and that the form of indication is such as to permit a pilot to readily detect deviations from the course which are so small as to be almost negligible.

While I have shown and described the preferred embodiment of my invention, I do not desire to be limited to any of the details of construction shown or described herein, except as defined in the appended claims.

I claim:

1. The method of producing a radio frequency equi-signal surface for guiding mobile vehicles which consists in simultaneously radiating radio frequency signals of like frequency from three aligned points spaced from each other, and modulating the signal radiated from the center one of said three points with a relatively low audible frequency wave comprising a sine wave fundamental and a lower amplitude sine wave second harmonic.

2. The method of producing a radio frequency equi-signal surface for guiding mobile vehicles which consists in simultaneously radiating radio frequency signals of like frequency from three aligned points spaced from each other, modulating the signal radiated from the center one of said three points with a relatively'low audible frequency wave, and reversing the phase of said modulated signal every half cycle of said audible frequency wave.

3. The method of producing a radio frequency equi-signal surface for guiding mobile vehicles which consists in simultaneously radiating radio frequency signals oflike frequency from three aligned points spaced from each other, modulating the signal radiated from the center one of said three points with a relatively low audible frequency wave of complex form characterized by a non-uniform time spacing between equal intensity peaks, and reversing the phase of said modulated signal every half cycle of said audible frequency wave.

4. The method of producing and using a radio frequency equi-signal surface for guiding a mobile vehicle along said surface which consists in simultaneously radiating radio frequency signals of like frequency from three aligned points spaced from each other, modulating the signal radiated from the center one of said three points with a relatively low audible frequency wave, reversing M the phase of said modulated signal every half cycle of said audible frequency wave, simultaneously receiving and rectifying the signals radiated from said three points, and observing the relative amplitudes of a'djacently disposed peaks of said audible frequency wave.

5. A radio beacon system for guiding a mobile vehicle along a selected course comprising a transmitting mechanism including a pair of spaced antennae, means for radiating from said antennae radio frequency course-defining signals of a given frequency and bearing such phase relation to each other as to extend a plane of minimum signal intensity in the direction of said selected course, a central antenna positioned midway between said pair of antennae, means for radiating from said central antenna radio frequency indicating signals having said given frequency and normally bearing along said course substantially a phase quadrature relation to said course-defining signals, means for modulating said indicating signals at a relatively low audible frequency, and means for reversing the phase of said indicating signals every half cycle of said audible frequency; and receiving and indicating means on said vehicle including means for simultaneously receiving said course-defining signals and said indicating signals, means for rectifying said signals, and a cathode ray tube coupled to visually indicate the wave form of said signals after rectification.

6. A radio beacon comprising three antennae spaced from each other, means for radiating simultaneously from said antennae radio frequency signals of like frequency, and means for modulating the signals from one of said antennae with a relatively low audible frequency Wave comprising a sine wave fundamental and a lower amplitude sine wave second harmonic.

'7. A radio beacon comprising three antennae spaced from each other, means for radiating simultaneously from said antennae radio frequency signals of like frequency, means for modulating the signals from one of said antennae with a relatively low audible frequency wave comprising a sine wave fundamental and a lower amplitude sine wave second harmonic, and means for reversing the phase of said modulated signals every half cycle of said audible frequency wave.

8. A radio beacon comprising three antennae spaced from each other, means for radiating simultaneously from said antennae radio frequency signals of like frequency, means for modulating the si nal from one of said antennae with a relatively low audible frequency wave of complex form characterised by a non-uniform time spacing between equal intensity peaks, and means for reversing the phase of said modulated signal every half cycle of said audible frequency wave.

WILLIAM J. OBRIEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 

